The present invention relates to magnetic resonance imaging and, in particular, to gradient coils used therewith.
A Magnetic Resonance Scanner where the direction of the main static field is orthogonal to the pole surface and to the magnet""s main symmetry plane is commonly called an Open Magnet System.
Previous methods for production of linear magnetic gradients in such MRI systems consist of winding discrete coils, in a bunched or distributed fashion on an electrically insulating hollow right cylinder coil-form, and driving said coil with a limited voltage. Conventional bunched coil designs are the Maxwell and Modified Maxwell Pair for the Z-gradient coil (parallel to the main magnetic filed), and the Golay or Modified Golay (multi-arc) Saddle coils for the X/Y gradient coils. Previous methods consisted of iteratively placing elemental loops or arcs on the cylindrical coil form until the desired gradient strength, gradient uniformity and inductance were achieved. In general, these types of designs constituted as method referred as a xe2x80x9cforward approachxe2x80x9d whereby a set of initial coil positions (i.e. initial current distribution) were defined. The gradient field, the energy/inductance were calculated and if they did not fall within specified design criteria, the positions of the coils will be shifted (statistically or otherwise) and the new outcome will be re-evaluated. This iterative procedure will continue until a suitable solution that satisfies all the design criteria is found.
An alternative approach in the design of gradient coils is referred as an xe2x80x9cinverse approachxe2x80x9d. According to this methodology, a set of predetermined constraint points for the gradient magnetic field is chosen inside the desired imaging volume. Via a Lagrange minimization technique involving the stored magnetic energy or power of the structure, the appropriate continuous current distribution that generates the predetermined gradient field inside the imaging volume is derived. Using a stream function discretization technique, the continuous current distribution of the coil can be approximated by a set of discrete current loops that are connected in series and share the same constant current. In order to ensure that the discretization process is valid, the gradient field is regenerated using the Biot-Savart law to the discrete current distribution and compare the results with the ones of the analytic approach. If the error is relatively small, the discrete current distribution is accepted as a valid one.
Open magnet geometries with vertically directed fields have been a successful presence in the MRI arena due to the inherent open architecture scheme that promotes patient comfort and diminishes patient claustrophobia. Recently, vertical field systems with higher main field strengths (0.5 T to 1.0 T) were also introduced in an attempt to improve image quality, as well as to expand the spectrum of clinical applications, especially fast imaging techniques not available on lower field strength MR scanners. The necessity for utilizing such imaging techniques is strongly dependent upon the availability of a suitable gradient system capable of delivering high gradient field strengths with a significant improvement to the gradient coil""s slew rate. For open magnet geometries with vertically directed fields, biplanar gradient coils have been the geometry of choice, since there are conformed to the main magnet""s geometric shape and assist in improving patient comfort and reducing patient claustrophobia. There is however a drawback in regards to the biplanar designs. Because of the demand that the coil must have enough gap to allow for clear access to the human torso, the distance between the two planes is excessively high. Increasing the gap between the two planes reduces the efficiency of the biplanar gradient in a level. Thus for a whole body gradient coil set, it is very difficult to generate gradient fields of the order of 50-60 mT/m with slew rates approaching 1000 T/m/sec. An alternative solution to this issue is the introduction of uniplanar gradient coil designs, which offer superb gradient field characteristics over a very limited imaging volume. Uniplanar gradients are mainly utilized on local imaging applications and are not recommended for applications with sizable FOV such as the one covering the human head. One viable alternative for performing ultra fast imaging of the human head for a vertical field system is the use of insertable biplanar gradients in a very close proximity to the human head. Due to the reduction of the gap between the planes the efficiency of the coil is significantly improved, and such a design is capable of generating high peak and high uniformity gradient fields over a volume similar to the size of the human head (25 cm DSV). But such a design is prohibitive because it violates the two principles of which the open magnet designs are based on. Placing the two planes over the human head and especially over the patient""s eyes, it compromises the patient""s comfort and enhances patient anxiety.
Gradient coil sets in common use make serious tradeoffs between coil performance and maintaining the xe2x80x9copen,xe2x80x9d non-claustrophobic configuration, desired in open MRI systems.
A MRI gradient coil set includes a uniplanar Z-gradient coil, a biplanar X-gradient coil, and a biplanar Y-gradient coil. The coil set provides an open Z-axis face.